A motor-vehicle door latch normally has a housing, a lock fork on the housing engageable with a door bolt and pivotable between a holding position engaged around the bolt and retaining it on the housing and a freeing position permitting the door bolt to move into and out of the housing, and a release pawl engageable with the fork and displaceable between a latched position retaining the fork in the holding position and an unlatched position unengageable with the fork and permitting the fork to move into the freeing position. An actuating mechanism is movable between an actuated position and an unactuated position and normally has an inside and an outside actuating lever connected to respective door handles. A coupling part is displaceable on the housing between a coupling position connecting the actuating mechanism to the release pawl for displacement of the release pawl into the unlatched position on displacement of the actuating mechanism into the actuated position and a decoupling position for disconnecting the actuating mechanism from the release pawl. Thus in the decoupling position actuation of the actuating mechanism does not affect the release pawl. A central locking element is displaceable on the housing between locked and unlocked positions and is connected via a locking mechanism normally also operable by at least an inside locking element with the coupling part for displacing the coupling part into the decoupling position on displacement of the central locking element into the locked position and for displacing the coupling part into the coupling position on displacement of the central locking element into the unlocked position. A drive body rotatable about a drive axis has an eccentric pin formation defining on rotation of the drive body an orbit lying partially inside and partially outside the cutout. A reversible electric motor rotates the drive body and thereby orbits the pin formation about the drive axis for moving the central locking element into the locked position and in the opposite direction for moving it into the unlocked position.
Thus such a latch can be locked or unlocked both manually or via the central-system motor. In particular it is important to be able to unlock a door when the central locking system has failed, for instance when the vehicle's battery has gone dead. In order to avoid having to move all the central-locking mechanism when thus manually unlocking the door, it is standard for the motor to reset to a neutral position after locking the door. In European patent document 0,267,423 of Hayakawa the central locking element is therefore a lever moved by the motor from the neutral position in one direction or the other to lock the door. A strong spring urges this lever back in to the neutral position and the motor is effective on the lever via a nonlocking worm drive, so that once the motor stops rotating the spring pulls the lever back to the neutral position. With such a system the spring must be strong enough to overcome the inherent resistance of the system and to back-drive the motor, and the motor must in its turn be strong enough to overcome the spring and the inherent resistance of the system. Hence the drive must be fairly bulky. Systems with similar operations and problems are seen in German 3,924,231, 4,009,276, 3,924,209, and 3,294,210 all of R. Fukumoto et al.
European 0,379,273 of S. Wilkes describes another system using a spiral drive that drive a pin mounted on a lever constituting the central locking element. At the end of its travel the lever cannot move, however, so that there is no possibility of a manual locking or unlocking of the latch. Although a spring return for manual unlocking in case of power failure is provided, it has the same disadvantage as the above-discussed prior-art systems.
In U.S. Pat. No. 4,709,738 of J. Ingenhoven an electrically operable central locking and unlocking device for vehicle doors is provided with electromechanical actuation for each lock. The actuation is transferred by a vertically movable actuating rod capable of moving between an unlocked position and a locked position to a control rod of the associated door lock. A reversible electric motor, transmission, and actuating mechanism are provided for the actuating rod. Exterior ridges on a spindle not and projecting formations which carry along the actuating rod during the lock stroke and during the unlocking stroke are provided. The ridges can rid, after completion of the lock stroke, over the projections as well as after completion of the unlock stroke. The spindle has a self-retarding thread on which is held the spindle nut that is formed with the ridges. Here also a very powerful motor must be provided to operate the lock.